Secondary battery comprising a positive electrode made of an electrically conductive polymer, an oxide of a transition metal or the like and a negative electrode made of metal lithium or a lithium alloy have been proposed for use in place of Ni—Cd battery and lead acid battery, to take advantage of high energy density.
These secondary batteries, however, have such a problem that the capacity drops significantly due to deterioration of the positive electrode or the negative electrode after repeated charges and discharges, resulting in performance unsatisfactory for practical use. Deterioration of the negative electrode, in particular, is accompanied by the generation of lichen-like lithium crystal called dendrite, that eventually penetrates a separator as charge and discharge cycles are repeated, resulting in short-circuiting in the battery and, in some cases, leads to a safety problem such as the explosion of the battery.
For the purpose of solving the problems described above, such a battery has been proposed as a carbon material such as graphite is used as the negative electrode and a metal oxide including lithium such as LiCoO2 is used as the positive electrode. This battery is the so-called rocking chair type battery which is charged after being assembled so as to supply lithium from the metal oxide including lithium of the positive electrode to the negative electrode and return lithium from the negative electrode to the positive electrode during discharge, and is called the lithium ion secondary battery distinguishing it from the lithium battery that employs metal lithium, since only lithium ions are involved in the charge and discharge processes, without using metal lithium in the negative electrode. This battery is characterized by high voltage, high capacity and high safety.
However, cycle life of the lithium ion secondary battery is said to be about 1000 cycles, since the metal oxide including lithium used as the positive electrode active material and graphite used as the negative electrode active material repeat expansion and contraction every time the battery is charged and discharged. Also because graphite of the negative electrode has a layered structure, response of lithium ions to quick charge or discharge (charge/discharge with a large current) is slow and metal lithium may precipitate on the surface of the negative electrode graphite during charging which poses a danger. Consequently, the separator is required to have a certain level of strength which is determined by needle piercing test. Efforts are also made in relation to the circuit so as to prevent large current from flowing.
The lithium ion secondary battery is used mainly in cellular phones and laptop computers, and is under demand to further increase the energy density. Measures studied for this purpose are mainly to increase the discharge capacity of the positive electrode and the negative electrode, improve the charge and discharge efficiency and increase the electrode density. When designing a cell in general, the thickness and density of each electrode are determined so that the charge stored in the positive electrode and the charge stored in the negative electrode become equal. As a result, discharge capacity of the cell is determined by the charge and discharge efficiency of the positive electrode or the negative electrode, whichever the lower, so that the higher the charge and discharge efficiency, the larger the cell capacity.
The present inventors are developing a lithium ion secondary battery that employs a polyacene-based organic semiconductor (hereinafter referred to as PAS), and have invented a secondary battery having a high energy density by the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-203833. The PAS used in the negative electrode of the present invention has higher capacity than a negative electrode made of graphite, but has charge and discharge efficiency as low as 60% to 80% which makes it impossible to achieve a high capacity of the cell by the same design method as that of the ordinary lithium ion secondary battery. Accordingly, the present inventors have achieved a higher capacity by causing lithium to be deposited on the negative electrode PAS in advance by the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-7928.
In the case of ordinary design, 100% of the negative electrode capacity can be utilized but only about 60% to 80% of the positive electrode capacity can be utilized. By having lithium deposited on the negative electrode PAS in advance, it is made possible to utilize 100% of the discharge capacity of both the positive electrode and the negative electrode, thus achieving higher capacity. At the 35th Battery Conference held in November 1994, the present inventors reported a lithium ion secondary battery having energy density as high as 450 Wh/l achieved by this method.
Recently as the environmental problems attract the public concerns, vigorous efforts have been made to develop renewable energy storage systems based on solar or wind power generation, distributed power sources intended to level off the load on the electric power supply, and power sources (main source and auxiliary source) for electric automobiles that would replace the gasoline-powered automobiles. While lead acid batteries have been used to power the electrical devices of the automobile, new power sources are called for in view of energy density and output power density, as the automobile comes to be equipped with ever increasing electric devices such as power window and stereo.
The lithium ion secondary battery described above has been researched as a promising power source having high capacity and there are already some products having been commercialized, but there remain problems related to safety, cycle life and output characteristic. Therefore, electric double layer capacitor (the rest is shortened to EDLC) has also been attracting attention. The EDLC is an electric device widely used as the power source for memory backup of IC and LSI, and is considered to have a position somewhere between the battery and the capacitor. The EDLC employs polarizing electrode based mainly on active carbon for both the positive electrode and the negative electrode, and is characterized primarily by high output characteristic and maintenance-free performance which are not achieved by the lithium ion battery and the nickel-hydrogen battery, as the EDLC has excellent instantaneous charge and discharge characteristic, although the discharge capacity per one charge is lower than that of a battery, and can endure ten to hundred thousand cycles of charge and discharge.
Although researches are being made on the application of the EDLC to the electric vehicle by taking advantage of the above-mentioned merits, it has not reach the practical level since typical energy density of the EDLC is around 3 to 4 Wh/l which is smaller by two factors than that of the lithium ion battery. It is said that energy density must be 6 to 10 Wh/l to be considered practical, and 20 Wh/l to become popular, for the application of the EDLC to the electric vehicle (Fujio MATSUI, 39th Electrical Chemistry Seminar, November 1999).
Capasitance C of the EDLC is given by C=∫ε/(4πδ)dS, where ε is the dielectric constant of the electrolytic solution, δ is the distance from the electrode interface to the ion center, and S is the surface area of the electrode interface. In order to make a capacitor having a high capacity and solve the problems described above, it is most important to pack a large amount of active material in a cell, namely to increase the bulk density of the electrode, and increase the specific surface area of the active material as will be evident from the equation shown above. However, there is a limit to the effect of increasing in the capacity by optimizing the micro-pore structure of the active material, since the specific surface area and the bulk density are inversely proportional to each other.
Another approach that can be conceived for increasing the capacity is setting the charge voltage higher. While the EDLC is charged typically at 2.5 V, energy density can be increased if the capacitor can be charged with ahigher voltage. If the capacitor can be charged at 3.3 V, for example, energy density can be increased 1.7 times since the amount of energy is proportional to the square of voltage. When the charge voltage is set higher, however, charge voltage of the positive electrode increases and causes the electrolytic solution to be oxidized and decompose, resulting in shorter lifetime and higher internal resistance due to the generation of gas, deterioration of the electrode and other cause, thus losing the advantages of the EDLC.
In the meantime, the present inventors have developed a capacitor that employs PAS for the positive electrode and the negative electrode and has been commercialized. Although this capacitor has a capacity higher than that of EDLC that employs active carbon, it does not satisfy the requirements for capacity required in the application to the electric vehicle.
The present inventors have also commercialized a coin type PAS capacitor which employs PAS having lithium deposited on the negative electrode in advance and has nominal voltage of 3.3 V.
However, this capacitor does not provide sufficiently high capacity despite the design to increase the withstanding voltage of a capacitor having 2.5 V specification to 3.3 V.
Specifically, capacitance of the 3.3 V class PAS capacitor that has been commercialized is comparable to that of the aforementioned capacitor (2.5 V) that employs PAS in the positive electrode and the negative electrode (for example, PAS621 which is a coin type capacitor comprising positive electrode and negative electrode that are made of PAS and a cell that is 6 mm in diameter and 2.1 mm in height has capacity of 0.3F, while PAS621L of the same size which employs positive electrode made of PAS and negative electrode made of PAS having lithium deposited thereon in advance has capacity of 0.36F, only 1.2 times the former), and has low output characteristic, thus unable to solve the aforementioned problems.